With the recent development of radio communication technology, networks to which a mobile station can be connected are diversified into various kinds. Examples of the networks to which a mobile station can be connected include (1) a third-generation circuit switching domain (CS (Circuit Switching) domain), (2) a third-generation packet switching network (PS (Packet Switching) domain), (3) a next-generation packet switching network (EPC (Evolved Packet Core)), (4) a wireless LAN, and the like.
On the assumption that a mobile station can be connected to multiple networks, a technique of switching networks to which the mobile station is connected (handover technique) has also been proposed (for example, Non-Patent Document 1).
For example, in such a case as where radio quality is deteriorated in the network to which the mobile station is connected, the switching of networks (handover) described above is performed.    Non-Patent Document 1: 3GPP TR23.882 V1.9.0 (Section 7.8.2)
Here, the mobile station generally has one radio for setting a radio connection between the mobile station itself and the network. Specifically, since the mobile station has only one radio, the mobile station cannot connect simultaneously to more than one network.
Therefore, in the case of performing a handover from one network to another network, a radio connection set between the one network and a mobile station needs to be disconnected before a radio connection is newly set between the other network and the mobile station.
As described above, in the handover, a radio connection needs to be disconnected first, and then a new radio connection needs to be set from the beginning. This leads to lengthening of a period of time for which the radio connection is disconnected.